Due to primarily federal regulations, engine manufacturers are being forced to reduce the amount of harmful compounds in the combustion exhaust. In the case of lean burning engines--including diesel and certain spark igniting engines--a hydrocarbon (HC) species must often be injected into the exhaust to effectively reduce the NOx concentrations. In other words, in combustion exhaust purification systems having an oxygen environment above 3% concentration, some type of reducing agent, usually a hydrocarbon compound, must be introduced into the exhaust in order to achieve acceptable reduction levels of NOx compounds. The HC may be introduced in many ways, but typically is continuously injected into the exhaust upstream from an appropriate deNOx catalytic converter. Unfortunately, in many real world applications, a continuous introduction of HC into the exhaust is not always the most practical. Continuous injection systems oftentimes also typically have great difficulty performing satisfactorily across the complete operating range of the engine. In addition, these continuous systems are often poorly suited for control by today's digital engine control systems.
What is needed is an exhaust gas purification system that is easily controlled, performs effectively across a complete operating range of the engine, and achieves satisfactory reduction of NOx compounds in the exhaust.